    Patent Document 1: JP-A 2001-302399    Patent Document 2: JP-A 2005-272516    Patent Document 3: JP-A 2007-12702    Patent Document 4: JP-A H11-12091    Patent Document 5: JP-A 2007-197382    Non-Patent Document 1: “Handbook of nanomaterials,” supervised by Toyoki Kunitake, NTS Inc., Feb. 25, 2005
Semiconductor particles generally have a property of emitting light having a wavelength inversely proportional to the difference of two energy levels by absorbing energy such as light or electron beam. This property has already been used in a fluorescent material for displays such as cathode-ray tubes (CRT) or plasma display panels (PDP). The semiconductor particles used for these displays are called bulk bodies and have a particle diameter of several μm (1 to 10 μm). If the particle diameter of the semiconductor particle is smaller than two times of Bohr radius (a0) thereof, such as 1 nm to several tens nm, degeneration of an energy band seen in a bulk semiconductor is solved and the orbit is discretized, a quantum size effect such that an energy width in a forbidden band changes depending on the particle diameter is seen, and the semiconductor particle comes to show a property different form that of a bulk body. Such semiconductor microparticles are called supermicroparticles, colloidal particles, nanoparticles, nanocrystals, quantum dots or the like, and industrial application as a new photo-functional material has been strongly demanded.
This quantum size effect makes the energy width of the forbidden band of a semiconductor nanoparticle decrease or increase, accompanied by the increase or decrease of the particle diameter. The change of the energy width of the forbidden band, for example, influences the fluorescent property of a particle. Those having a small particle diameter and a wide energy width of a forbidden band have a fluorescent wavelength in the shorter wavelength side, and those having a large particle diameter and a narrow energy width of a forbidden band have a fluorescent wavelength in the longer wavelength side. That is, semiconductor nanoparticles have also been attracting attention as materials capable of creating an arbitrary fluorescent color by controlling their particle diameters.
Such semiconductor microparticles have been actively studied from both basic and applied aspects as a new functional substance group. Of these semiconductor microparticles, many researchers and engineers have begun to be interested in semiconductor nanoparticles because of a report of red light emission from porous silicon at a room temperature in 1990, and basic understandings of the semiconductor nanoparticles can be mainly obtained from silicon nanoparticles or nanocrystals including porous silicon. Of group IV semiconductors such as silicon, germanium, carbon and tin, silicon is particularly characteristic of non-toxic, existing in abundance and being a semiconductor comprised of only a single element. Further, silicon is a substance having attracting characteristics excellent in thermal stability and capable of forming a surface oxide film in good quality, and is a key substance in electronics. However, nanosemiconductors, as fluorescent substances widely applied to plasma display panels or fluorescent lamps, bio-related substances such as antigens, antibodies and DNA, or marker substances for highly sensitively measuring environmental-related substances such as dioxins, etc. and intended for applications to optical information and communication field, photonics, that is as an emitting material, are mainly comprised of group III-V compounds, and studies of silicon-based semiconductor materials including silicon (germanium, carbon and tin) have not been proceeding so much. The main reason is that silicon itself is not an emitting material (Non Patent Document 1 and Patent Document 3).
In group III-V compound semiconductors used in the field of an optical device, electrons and holes excited in direct transition semiconductors are directly recombined to emit light. On the other hand, single element semiconductors such as silicon, germanium and carbon are indirect transition semiconductors, phonons are required for radiative recombination transition according to the law of conservation of momentum, and the radiative recombination probability is small. Therefore, various silicon material groups including nanoparticles are targeted in order to obtain optical functionality not contained in bulk crystalline silicon. Previously, in a method for emitting silicon, nanoparticles, nanocrystals, porous silicon, rare earth ions-added bulk crystals, mixed crystals (SiGe, SiGeC, SiC), superlattice structures (Si/Ge, Si/SiO2), amorphous silicon, thermal silicon oxides, organic silicon compounds, semiconductor silicides and the like are targeted for the study, and in particular, nanoparticles exhibit high emission efficiency at a room temoerature and have been attracting attention as emitting materials.
The exciton Bohr radii of crystal silicon and germanium are about 5 nm and about 18 nm, respectively. In order to obtain luminescent property different from that of a bulk crystal due to quantum confinement effect, nanoparticles having a size of about exciton Bohr radius needs to be produced, and the size of nanoparticles of silicon or germanium exhibiting visible luminescence needs to be less than 20 nm.
Conventionally, silicon, silicon carbide, silicon germanium and the like are mainly used for semiconductor materials constituting solar cells. These semiconductor microparticles have different properties depending on their particle diameters when also used for films such as an active layer of a film transistor and a silicon layer of a solar cell. Further in nanoparticles, nanocrystals, and porous silicon that have been studied for using silicon as an emission material described above, it is difficult to control the structures and particle diameters of nanoparticles due to an uneven reduction reaction when they are synthesized in a liquid phase. In bulk crystals and nanocrystals with which rare earth ions such as Er ions are added (doped), since the rare earth ions such as luminescent Er are not very soluble in silicon crystals, and as a result, it is difficult to obtain an emission. Further, merely those having large particle diameters can be produced by vapor phase cluster synthesis. In other materials of silicide semiconductors such as FeSi2, amorphous silicon and thermal silicon oxides, at present there is no material that can allow silicon to emit efficiently (Patent Document 3).
In order to employ the properties of semiconductor microparticles described above, it is very important to produce semiconductor microparticles having a uniform size.
Further, it is very important to produce silicon-based semiconductor microparticles having a uniform size of 20 nm or less in order to be capable of imparting luminescent function to the silicon-based semiconductor microparticles including silicon, and to exert the properties efficiently. A method for strictly and separately producing the particle diameter and the crystal form (such as amorphous) is needed.
Examples of the method for producing semiconductor microparticles include various methods such as a high vacuum process including a molecular beam epitaxy method or a CVD method, a reverse micelle method, a hot soap method, and methods employing acid-base reactions. However, it is difficult to obtain semiconductor microparticles having a uniform size even using these production methods, and therefore, compound semiconductor microparticles having a uniform size are obtained according to a method of conducting the crystal growth of semiconductor microparticles under light shielding conditions as described in Patent Document 1, a method in which a solution from which a compound semiconductor is separated is further subjected to heat treatment at 50° C. to 300° C. as described in Patent Document 2, or the like.
However, in these production methods, the semiconductor particles after reaction require a particular post-treatment or process, thereby requiring complication of the steps or a long-term reaction and operation in production. Further, all these problems are due to a side reaction caused by photoreaction or a side reaction caused by temperature ununiformity or ununiformity of stirring in a reactor upon obtaining semiconductor microparticles. Therefore, homogenization of the particle size and unification of a reaction product can be effectively and efficiently carried out by suppressing such side reactions.
It was difficult to obtain silicon-based semiconductor microparticles having a uniform size of 20 nm or less in the production methods. As described in Patent Document 4, there is a method in which the circumference of the spherical polycrystal silicon produced by a high frequency thermal plasma method is covered with a thermal oxide film, and a part of the spherical polycrystal silicon covered with a thermal oxide film is heated and melted to carry out recrystallization while shifting the molten moiety, and the like, however, complication of the steps and enlargement of a device are inevitable, and there is a problem such as an increase of a burden on environment due to an increase of a power used.
When semiconductor microparticles having a uniform size are produced by a liquid-phase method, all conditions in the reactor are required to be uniform. Further, when the size of the particle is nanometer order, control for particularly strict reaction conditions is required. Therefore, a method for obtaining semiconductor microparticles by conducting a reaction in a flow path having a width of 1 mm or less called microchemical process as described in Patent Document 5 is known. Specifically, a method in which a microreactor or micromixer is used is known. It is said that conduct of a reaction in a microflow path in a microreactor enhances mixing rate and efficiency to improve the concentration and temperature uniformity in the reaction conditions in the flow path, thereby carrying out homogenization of the particle size and unification of a reaction product effectively and efficiently through suppression of the side reaction and the like. However, since the product or the foam or by-product generated by the reaction clogs the flow path, which causes with high probability clogging in the micro flow path, and basically, since the diffusion of molecules only promotes the reaction, the process is not applicable to all reactions. The microchemical process uses a scale-up method of increasing the number of small reactors arranged in parallel, but there is a problem that because the manufacturing ability of one reactor is small, large scale up is not practical, and the respective reactors are hardly supplied with the same performance, thus failing to provide uniform products. When the reaction solution is highly viscous or the reaction causes an increase in viscosity, a very high pressure is necessary for passage of the solution through a minute flow path, so there is a problem that a usable pump is limited, and leakage of the solution from an apparatus cannot be solved due to high pressure.
In light of the above, it is an object of the present invention to provide a method for producing compound semiconductor microparticles obtained in a thin film fluid formed between two processing surfaces arranged to be opposite to each other to be able to approach to and separate from each other, at least one of which rotates relative to the other, wherein the temperature in the thin film fluid is highly uniform and stirring in a reaction container is highly uniform, so that compound semiconductor microparticles that are monodisperse according to the intended object can be prepared without high pressure and clogging with products due to self-dischargeability, achieving high productivity.
The present invention is based on whole new concept microchemical process technology in which the objects and problems of the conventional technique called “microchemical process technology” have been solved, and more specifically, it is an object of the present invention to provide a method for producing compound semiconductor microparticles obtained in a thin film fluid formed between two processing surfaces arranged to be opposite to each other to be able to approach to and separate from each other, at least one of which rotates relative to the other, wherein the temperature in the thin film fluid is highly uniform and stirring in a reaction container is highly uniform, so that compound semiconductor microparticles that are monodisperse according to the intended object can be prepared, clogging with products does not occur due to self-dischargeability, a large pressure is not necessary, and productivity is high.